Josephson junction readout for graphene-based single photon detector

ABSTRACT

A detector for detecting single photons of infrared radiation. In one embodiment a waveguide configured to transmit infrared radiation is arranged to be adjacent a graphene sheet and configured so that evanescent waves from the waveguide overlap the graphene sheet. In some embodiments the waveguide is omitted and infrared light propagating in free space illuminates the graphene sheet directly. A photon absorbed by the graphene sheet from the evanescent waves heats the graphene sheet. The graphene sheet is coupled to the weak link of a Josephson junction, and a constant bias current is driven through the Josephson junction, so that an increase in the temperature of the graphene sheet results in a decrease in the critical current of the Josephson junction and a voltage pulse in the voltage across the Josephson junction. The voltage pulse is detected by the pulse detector.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation of U.S. application Ser. No.14/846,013, filed Sep. 4, 2015, entitled “JOSEPHSON JUNCTION READOUT FORGRAPHENE-BASED SINGLE PHOTON DETECTOR”, the entire content of which isincorporated herein by reference, which claims priority to and thebenefit of U.S. Provisional Application No. 62/181,195, filed Jun. 18,2015, entitled “JOSEPHSON JUNCTION READOUT FOR GRAPHENE-BASED SINGLEPHOTON DETECTOR”, the entire content of which is incorporated herein byreference.

BACKGROUND

1. Field

One or more aspects of embodiments according to the present inventionrelate to detection of photons, and more particularly to a detector fordetecting individual photons of infrared light using a waveguide coupledto a graphene sheet, coupled, in turn, to a Josephson junction.

2. Description of Related Art

Detectors capable of detecting single photons have multipleapplications, including applications in quantum communications. Suchdetectors, for high-energy photons, may be constructed according to avariety of designs. For low energy photons, such as photons withwavelengths of 1 micron or more, however, there is a gap in detectortechnology. In particular, existing communications systems may use awavelength of 1550 nm, and other components, such as lasers, designed tooperate at this wavelength may be readily available, resulting inapplications for detectors operating at the same wavelength. Thus, thereis a need for a single-photon detector for low-energy photons.

SUMMARY

Aspects of embodiments of the present disclosure are directed toward adetector for detecting single photons of infrared radiation. In oneembodiment a waveguide configured to transmit infrared radiation isarranged to be adjacent a graphene sheet and configured so thatevanescent waves from the waveguide overlap the graphene sheet. In someembodiments the waveguide is omitted and infrared light propagating infree space illuminates the graphene sheet directly. A photon absorbed bythe graphene sheet from the evanescent waves heats the graphene sheet.Part of the graphene sheet is part of the Josephson junction as the weaklink, and a constant bias current is driven through the Josephsonjunction; an increase in the temperature of the graphene sheet resultsin a decrease in the critical current of the Josephson junction and avoltage pulse in the voltage across the Josephson junction. The voltagepulse is detected by the pulse detector.

According to an embodiment of the present invention there is provided aphoton detector including: a graphene sheet configured to absorbinfrared electromagnetic waves and to undergo an increase in atemperature of the graphene sheet when a photon of the infraredelectromagnetic waves is absorbed by the graphene sheet; a Josephsonjunction on the graphene sheet, the Josephson junction having a gapcoupled to electrons of the graphene sheet; and a circuit connected totwo contacts of the Josephson junction, the circuit configured to detecta decrease in a critical current of the Josephson junction, the decreasecorresponding to the increase in the temperature of the graphene sheet.

In one embodiment, the graphene sheet has an electron mobility of morethan 100,000 cm²/V/s.

In one embodiment, the graphene sheet has an electron mobility of about1,000 cm²/V/s.

In one embodiment, the graphene sheet substantially has the shape of arectangle, the rectangle having a length and a width, the length beinggreater than or equal to the width.

In one embodiment, the length of the rectangle is less than 20 microns.

In one embodiment, the product of the length of the rectangle and thewidth of the rectangle is less than 1000 square microns.

In one embodiment, the photon detector includes a first layer ofhexagonal boron nitride immediately adjacent a first surface of thegraphene sheet, and a second layer of hexagonal boron nitrideimmediately adjacent a second surface of the graphene sheet.

In one embodiment, each of the first layer of hexagonal boron nitrideand the second layer of hexagonal boron nitride has a thickness greaterthan 4 nm and less than 40 nm.

In one embodiment, the circuit includes a current source connected tothe two contacts of the Josephson junction and configured to drive aconstant bias current through the Josephson junction.

In one embodiment, the photon detector includes an amplifier connectedto the two contacts of the Josephson junction.

In one embodiment, the photon detector includes a matching circuitconnected between the two contacts and the amplifier.

In one embodiment, the photon detector includes a pulse detectorconnected to the amplifier, the pulse detector including a voltagereference and a comparator configured to compare a signal at an outputof the amplifier to a voltage at an output of the voltage reference.

In one embodiment, the graphene sheet consists of a single atomic layerof graphene.

In one embodiment, the graphene sheet includes two atomic layers ofgraphene.

In one embodiment, the photon detector includes a refrigeratorconfigured to cool the graphene sheet to a temperature below 4 K.

In one embodiment, the refrigerator is a pulse tube refrigerator.

In one embodiment, the refrigerator is a Gifford-McMahon cooler.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and embodiments are described in conjunction with theattached drawings, in which:

FIG. 1 is a block diagram of a single photon detector, according to anembodiment of the present invention;

FIG. 2A is a perspective view of a sensor assembly, according to anembodiment of the present invention;

FIG. 2B is a schematic side view of a graphene sheet sandwiched betweentwo layers of hexagonal boron nitride, according to an embodiment of thepresent invention;

FIG. 3A is a graph of voltage across a Josephson junction and currentflowing through the junction at a first temperature, according to anembodiment of the present invention;

FIG. 3B is a graph of voltage across a Josephson junction and currentflowing through the junction at a second temperature, higher than thefirst temperature, according to an embodiment of the present invention;

FIG. 4 is a block diagram of a pulse detector according to an embodimentof the present invention;

FIG. 5 is a schematic plan view of a sensor assembly according to anembodiment of the present invention;

FIG. 6 is a perspective view of a communications system including mobiletransceivers according to an embodiment of the present invention; and

FIG. 7 is a graph of critical current probability distributionsaccording to an embodiment of the present invention.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of aJosephson junction readout for a graphene-based single photon detectorprovided in accordance with the present invention and is not intended torepresent the only forms in which the present invention may beconstructed or utilized. The description sets forth the features of thepresent invention in connection with the illustrated embodiments. It isto be understood, however, that the same or equivalent functions andstructures may be accomplished by different embodiments that are alsointended to be encompassed within the spirit and scope of the invention.As denoted elsewhere herein, like element numbers are intended toindicate like elements or features.

Referring to FIG. 1, in one embodiment, a single-photon detector forinfrared light includes an infrared waveguide 110, a graphene sheet 120,and a Josephson junction 130. The infrared waveguide 110, graphene sheet120, and Josephson junction 130 form a sensor assembly, configured toreceive photons, and to produce electrical signals when the photons aredetected. In the sensor assembly, a photon may be absorbed by thegraphene sheet 120 from the waveguide 110; the photon heats the graphenesheet 120, which is coupled to the Josephson junction 130, and theheating of the graphene sheet 120 causes a decrease in the criticalcurrent 310 of the Josephson junction 130. When the critical current 310falls below a bias current 305 driven through the Josephson junction 130from a bias circuit 138, the voltage across the Josephson junction 130increases, i.e., a voltage pulse is produced. An amplifier 135 amplifiesthe voltage pulse, and a pulse detector 140 detects the voltage pulse.The combination of the Josephson junction 130, the bias circuit 138, theamplifier 135, and the pulse detector 140 forms a Josephson junctionreadout for detecting changes in the temperature of the graphene sheet120.

Referring to FIG. 2A, the waveguide 110 and the graphene sheet 120 ofthe sensor assembly may be fabricated as an integrated component on asubstrate 210. When infrared electromagnetic waves propagate in thewaveguide 110, evanescent waves outside the waveguide 110 overlap with,and couple to, the graphene sheet 120. This coupling allows photons ofthe electromagnetic waves to be absorbed by the graphene sheet 120,raising the temperature of the graphene sheet 120. When an infraredphoton is absorbed by the graphene sheet 120, the absorption occursprimarily through interaction of the photon with the electronic degreesof freedom of the graphene sheet 120, the interactions between thephoton and the nuclei of the graphene sheet 120 being significantlyweaker. Electrons in the graphene sheet 120 are weakly coupled tophonons in the graphene sheet 120, and the total heat capacity of theelectrons in the graphene sheet 120 is relatively small, so that theabsorption of a photon results in a relatively large and rapid increasein the electron temperature. Moreover, the electrons in the graphenesheet 120 are strongly coupled to each other, and, as a result, theelectron temperature within the graphene sheet 120 quickly becomessubstantially uniform after the absorption of a photon. As used herein,the temperature of the graphene sheet 120 refers to the temperature ofthe electrons in the graphene sheet 120; immediately after theabsorption of a photon, the electron temperature may differ from thephonon temperature.

The waveguide 110 may be fabricated by any of various methods known inthe art for fabricating a waveguide 110 on the surface of a substrate210. In one embodiment, a layer of silicon dioxide is formed on asilicon substrate 210, and patterned using photolithography to form thecore of the waveguide 110 as a raised structure. A layer 220 of siliconnitride may then be formed over the waveguide 110 and the surroundingarea, so that the waveguide 110 core has silicon nitride on both sidesof it and above it. This structure may then be polished, so that theupper surface of the structure is flat and smooth. In other embodiments,the waveguide 110 may be formed by depositing a layer of silicon dioxideon a silicon substrate 210, depositing a layer of silicon on the silicondioxide, and patterning the silicon layer using photolithography to formthe waveguide 110 as a raised silicon structure. This structure may thenbe planarized, i.e., made flat and smooth, by depositing an additionallayer of silicon dioxide and polishing it down to the top surface of theraised silicon structure. In other embodiments the waveguide 110 may becomposed of another first material, surrounded by one or more othermaterials having a lower index of refraction than the first material.The resulting waveguide structure may have a thickness of between 50 nmand 2000 nm; in one embodiment it has a thickness of between 100 nm and1000 nm. The transverse dimensions of the waveguide structure may besomewhat smaller or considerably smaller than the wavelength of theinfrared light to be detected. The waveguide 110 may be single-mode ormulti-mode when guiding light at the wavelength of the infrared light tobe detected.

The substrate 210 may be substantially flat, e.g., within 1 micron ofbeing flat over the area including the waveguide, and the waveguide maybe formed, e.g., using one of the processes described above, in a layerhaving a thickness greater than 50 nanometers and less than 2 microns.The front end of the waveguide 110 may extend to the edge of thesubstrate 210 as shown, and off-chip coupling optics 240 may be used tolaunch infrared light into the waveguide 110. In other embodimentsportions of the coupling optics 240 may be fabricated on the substrate210. In some embodiments the waveguide 110 is omitted and infrared lightpropagating in free space illuminates the graphene sheet 120 directly.

The Josephson junction 130 may be composed of two adjacentsuperconducting patches 250, deposited on the substrate 210 at the edgeof the graphene sheet 120, so as to overlap the edge of the graphenesheet 120. Each superconducting patch 250 may have a thickness in therange of 50 nm to 100 nm. The superconducting patches 250 may beseparated by a small gap 260, having a width between 100 nm and 1micron, e.g., 500 nm; this gap may act as a weak link. A diagnosticcontact 230 may also be deposited on the substrate 210, at an oppositeedge of the graphene sheet 120. Electrons in the two superconductingpatches 250 are coupled by tunneling across the gap 260. The Josephsonjunction 130 overlaps the edge of the graphene sheet 120, and electronsin the graphene sheet 120 couple to the gap 260 of the Josephsonjunction 130. The wave functions of electrons in the two superconductingpatches 250 extend into the gap 260 and as a result are coupled to theelectrons of the graphene sheet 120, the wave functions of which alsoextend into the gap 260. Because of this coupling, the temperature ofthe electrons of the graphene sheet 120 affects the critical current 310(FIG. 3) of the Josephson junction 130; the higher the temperature ofthe electrons of the graphene sheet 120, the lower the critical current310 of the Josephson junction 130. The superconducting patches 250 maybe located on the graphene sheet 120 at a position that is separated(e.g., separated by at least 1 micron) from the waveguide 110 to avoiddirection interaction of photons in the waveguide with thesuperconducting patches 250; such interaction may otherwise destroy thesuperconductivity of the superconducting patches 250. Thesuperconducting patches 250 may be formed of any of a number ofmaterials known in the art that become superconductive at lowtemperatures, including niobium nitride, niobium titanium nitride,niobium diselenide, aluminum, niobium, niobium titanium, or lead. Thecontacts formed between the superconducting patches 250 and the graphenesheet 120 may be highly transparent (i.e., they may have low contactresistance); each contact may be a one-dimensional (1D) contact along arespective edge of the graphene sheet 120, formed in a manner describedin further detail below. The superconducting materials employed in theJosephson junction 130, and other design parameters of the Josephsonjunction 130 may be selected to form a Josephson junction 130 with acritical temperature of the order of (i.e., comparable to) thetemperature change produced in the graphene sheet 120 by the absorptionof a photon. The lateral dimensions of the superconducting patches 250may be selected to be greater than the lateral extent of the Cooperpairs formed in the superconducting patches 250 when they are in asuperconducting state.

The two superconducting patches 250 of the Josephson junction 130 alsoserve as contacts. Conductors, e.g., gold wire or gold ribbon, may bebonded to the two superconducting patches 250 to connect the Josephsonjunction 130 to bias and readout circuitry. The bias and readoutcircuitry includes a bias circuit 138, configured as a current source,that drives a constant bias current 305 through the Josephson junction130. The bias circuit 138 may include an operational amplifier 135configured as a current source. Referring to FIG. 3A, the Josephsonjunction 130 has a current-voltage (I-V) curve with a superconductingportion corresponding to currents with a magnitude less than a criticalcurrent 310 within which the voltage across the Josephson junction 130is zero. For currents exceeding the critical current 310, the voltageacross the Josephson junction 130 is proportional to the current. Thebias current 305 is selected to be less than the critical current 310 atthe steady-state temperature of the graphene sheet 120 when no photonshave been recently absorbed. Referring to FIG. 3B, when a photon isabsorbed, the temperature increases, the critical current 310 decreasesto a decreased critical current 315, and, if the decreased criticalcurrent 315 is less than the bias current 305, the voltage increases toa value proportional to the bias current 305.

After absorption of a photon, the temperature of the graphene sheet 120initially increases as described above, and then decreases as theelectrons of the graphene sheet 120 lose heat energy through severalmechanisms, including coupling through the contacts (e.g., thediagnostic contact 230, and the two superconducting patches 250)coupling to the lattice, and coupling to the electromagneticenvironment. As such, after a photon is absorbed, the temperature mayincrease and then decrease again, and as a result, the critical current310 may decrease and the increase, and the voltage across the Josephsonjunction 130 may increase and then decrease, i.e., the Josephsonjunction 130 may produce a voltage pulse. The voltage pulse may beamplified by an amplifier 135 (FIG. 1), which may have sufficientbandwidth, e.g., a bandwidth of at least 100 MHz, or a bandwidth of 1GHz or more, to amplify the voltage pulse, which may be of shortduration because of the low heat capacity of the electrons of thegraphene sheet 120. The amplifier 135 may include a quantum noiselimited amplifier followed by a high electron mobility transistor (HEMT)amplifier. The Josephson junction readout may also include a matchingnetwork, e.g., an inductor-capacitor (LC) matching network, fortransforming the impedance of the Josephson junction 130 to the inputimpedance of the amplifier 135 (which may be about 50 ohms). In someembodiments the quantum noise limited amplifier may be a radio frequencysuperconducting quantum interference device (RF SQUID) amplifier, or itmay be a travelling wave parametric amplifier, or a tuned systemparametric amplifier (TPA), or any other kind of amplifier with asuitable frequency response that is quantum noise limited or nearlyquantum noise limited. In some embodiments the amplifier 135 does notinclude quantum noise limited amplifier, and has a HEMT amplifier as thefirst stage instead. The behavior of the Josephson junction 130 may beunderstood as the behavior of a phase particle in a tilted “washboard”potential in the RCSJ model. In some embodiments the Josephson junction130 may exhibit a behavior referred to as latching on, that may occurwhen the phase particle remains in a local minimum of the potential, andthat may result in the voltage across the junction not returning to zeroafter the absorption of a photon. In such a case, the readout circuitmay be configured to detect latching on (for example, when the durationof a pulse exceeds a threshold), and the readout circuit may then resetthe Josephson junction 130 by briefly setting the bias current to zero,causing the Josephson junction to return to a zero-voltage state, in astate change referred to as retrapping. The likelihood that theJosephson junction 130 will exhibit latching on behavior may be affectedby the magnitude of the bias current; e.g., a Josephson junction 130biased with a bias current near the critical current 310 may be morelikely to exhibiting latching on behavior than a Josephson junction 130biased with a bias current well below the critical current 310.

A pulse detector 140 connected to the Josephson junction 130 may be usedto detect the absorbed photons. Referring to FIG. 4, the input to thepulse detector 140 may be a voltage from the output of the Josephsonjunction 130, which may be zero when no photons have been recentlyabsorbed, and which may exhibit a voltage pulse when a photon isabsorbed. The pulse detector 140 may have a set threshold voltage, andit may generate a digital output pulse, or increment a photon countregister, each time the input voltage increases past the thresholdvoltage, i.e., crosses the threshold voltage with a positivetime-derivative. For example, the pulse detector 140 may include acomparator 410, one input of which is (or is connected to) the input ofthe pulse detector 140, a second input of which is connected to avoltage reference 420 that defines the threshold voltage, and the outputof which is connected to a digital processing circuit 430. For examplethe output of the comparator 410 may be connected to the clock input ofa flip-flop or other edge-triggered circuit, so that the detection of aphoton causes an essentially immediate change of state in the processingcircuit 430.

The waveguide 110 may be straight, and, to increase the amplitude of theevanescent waves overlapping the graphene sheet 120, it may be part ofan optical resonator, constructed, for example, by forming a reflector(e.g., a Bragg reflector) at each end of a section of the waveguide 110.Bragg reflectors may be formed by creating periodic defects in oradjacent the waveguide 110, e.g., by forming holes in or adjacent thewaveguide structure with a focused ion beam. The reflector at the frontend of the waveguide 110 (i.e., the end first encountered by thearriving infrared light) may be partially reflective, to allow theinfrared light to enter the resonator, and the reflector at the otherend (the “back” end) of the waveguide 110 may be highly reflective, toavoid allowing light to escape from the back end of the waveguide 110.In some embodiments only one reflector is used, at the back end of thewaveguide 110.

In other embodiments the waveguide 110 may not be straight, but may haveone or more curves, increasing the length of the section of waveguide110 that is adjacent the graphene sheet 120, and from which evanescentwaves may interact with the graphene sheet 120. A curved section in thewaveguide may have a radius of curvature less than the length of thegraphene sheet 120, and in the curved section the direction of thewaveguide may change by 45 degrees or more. The curvature of thewaveguide may at all points be sufficiently gradual, e.g., having aradius of curvature exceeding the wavelength in the waveguide, of theinfrared light, to avoid excessive loss. The increased length of thesection of waveguide 110 adjacent the graphene sheet 120 may increasethe probability, for any photon launched into the waveguide 110, ofbeing absorbed by the graphene sheet 120. The waveguide 110 may have adouble spiral shape as illustrated in FIG. 2A. In other embodiments thewaveguide 110 may have the shape of a single spiral, with the back endof the waveguide in the center of the spiral. The back end of thewaveguide 110 may be at an edge of the substrate 210 as illustrated inFIG. 2A, or it may be elsewhere on the substrate 210, e.g., near themiddle in the case of a waveguide 110 in the shape of a single spiral.Referring to FIG. 5, in one embodiment the waveguide 110 may have ameandering shape, covering a region under a portion of the graphenesheet 120 as illustrated. In yet other embodiments, the waveguide 110may have one or more curves and also form part of a resonator, tofurther increase the absorption probability for photons launched intothe waveguide 110.

Infrared light may be launched into the waveguide 110 by any of severalsystems known to those of skill in the art. For example, if the infraredlight to be detected is initially travelling in free space, it may becoupled into the waveguide 110 using one or more suitable poweredoptics, such as lenses or curved mirrors. Referring to FIG. 6,transceivers using coupling to free-space propagating waves may be usedfor mobile communications e.g., between ships 680 and a tower 685. Ifthe infrared light to be detected is initially propagating in an opticalfiber, it may be launched into the waveguide 110 of the single photondetector using any suitable one of a variety of fiber-to-chip couplersknown to those of skill in the art.

The graphene sheet 120 may be a single-layer sheet, i.e., it may be oneatomic layer thick, or it may be a multi-layer graphene sheet 120,having, e.g., 2, 3, 4, or more layers. Referring to FIG. 2B, in oneembodiment, the graphene sheet 120 is encapsulated in hexagonal boronnitride (hBN). As is known to those of skill in the art, a sandwich isformed, with the graphene sheet 120 sandwiched between two layers 290 ofhexagonal boron nitride. Each layer 290 of hexagonal boron nitride maybe between 4 nm and 40 nm thick; these layers 290 of hexagonal boronnitride may keep the surface of the graphene sheet 120 clean, i.e., theymay prevent surface contamination from compromising the properties ofthe graphene sheet 120. The sandwich, composed of the two outer layers290 of hexagonal boron nitride encapsulating the graphene sheet 120, maythen be disposed on the portion of the substrate 210 that includes thewaveguide 110.

Each hexagonal boron nitride layer 290 may be a single crystal, with anatomically flat surface facing the graphene sheet 120. Each hexagonalboron nitride layer 290 may be annealed, e.g., at 250° C. for 10-15minutes, before the sandwich is assembled. The sandwich may be formed byfirst bringing a first layer 290 of hexagonal boron nitride into contactwith the graphene sheet 120, resulting in adhesion of the graphene sheet120 to the hexagonal boron nitride by van der Waals forces, and thenbringing the graphene sheet 120, on the first layer 290 of hexagonalboron nitride, into contact with the second layer 290 of hexagonal boronnitride, resulting in adhesion, again by van der Waals forces, at theinterface between the graphene sheet 120 and the second layer 290 ofhexagonal boron nitride. The edges of the sandwich may then be etched,e.g. using plasma etching, so that the edges of the two layers 290 ofhexagonal boron nitride and the edges of the graphene sheet 120 in thestructure remaining after the etch process coincide (i.e., are aligned).

The graphene sheet 120 may be rectangular as illustrated in FIG. 2A,with a length and a width, the length being greater than or equal to thewidth. The total area of the graphene sheet 120 may be less than 1000square microns. In one embodiment the graphene sheet 120 is about 10microns by 10 microns. In one embodiment the graphene sheet 120 has alength in the range 1.0-100.0 microns and a width in the range 1.0-100.0microns. An electrical contact 230 may be provided at one end of thegraphene sheet 120. In one embodiment, contact is made with the edge ofthe graphene sheet 120, using a layer of metal deposited onto the edgeof the sandwich.

For good performance, the graphene sheet 120 may be made as small aspossible, kept as clean as possible, and operated at as low atemperature as possible. In one embodiment, the graphene sheet 120 iscooled to 4 K, using, for example, a pulse tube refrigerator or aGifford-McMahon (GM) cooler. In other embodiments direct cooling withliquid helium, or with liquid helium in a partial vacuum (e.g., using a1 K pot, to reach temperatures below 4 K) may be used to cool thegraphene sheet 120.

Diagnostic circuitry may be connected to two contacts at opposite endsof the graphene sheet 120, e.g., to the diagnostic contact 230 and toone of the superconducting patches 250. The diagnostic circuitry mayinclude components and connections that may be used for diagnostics,e.g., during manufacturing, operation, or service. For example, a biastee may be used to drive a low-frequency current through the graphenesheet 120, modulating its temperature, and the presence of acorresponding modulation at the output of a power detector measuring thethermal noise (or “Johnson noise”) power at radio frequencies or atmicrowave frequencies may then be used to measure the differentialthermal conductance or to verify the functioning of the devicegenerally. The superconducting patches 250 are connected to the graphenesheet 120 electrically but not thermally. The Cooper pairs insuperconductors do not conduct heat.

The thermal activation and the macroscopic quantum tunneling of thephase particle of the Josephson junction, both of which may in principlebe sources of error, are exponentially suppressed and negligibly smallwhen the bias current and temperature are small and low enough,respectively.

FIG. 7 shows a first probability distribution 710 of the peak voltage atthe input of the pulse detector 140 (e.g., the voltage at the output ofthe amplifier 135) after a photon is absorbed, and a second probabilitydistribution 720 of the voltage at the input of the pulse detector 140in the absence of absorbed photons. The threshold voltage (“V_(thres)”)of the pulse detector 140 may be adjusted to adjust both the dark countrate and the quantum efficiency: if the threshold voltage is increased,the dark count rate and the quantum efficiency are both decreased,because the likelihood that amplifier noise will trigger a count isreduced, and the likelihood that a pulse resulting from the absorptionof a photon fail to trigger a count is increased. Conversely, if thethreshold voltage is decreased, the dark count rate and the quantumefficiency are both increased, because the likelihood that amplifiernoise will trigger a count is increased, and the likelihood that a pulseresulting from the absorption of a photon will trigger a count is alsoincreased.

The greater the separation between the first probability distribution710, and the second probability distribution 720 (relative to the widthsof these distributions), the higher the achievable quantum efficiencyfor a certain dark count rate will be, or, equivalently, the lower theachievable dark count rate for a certain quantum efficiency will be. Theseparation between the probability distributions may be affected by thesize of the graphene sheet 120, the cleanliness of the graphene sheet120, and the temperature of the graphene sheet 120 in the absence ofabsorbed photons. The smaller and cleaner the graphene sheet 120 is, andthe lower its temperature, the greater will be the separation betweenthe first probability distribution 710 and the second probabilitydistribution 720. The cleanliness of the graphene sheet 120 may bequantified as an impurity density level, with a cleaner graphene sheet120 having a lower impurity density level, or as an electron mobility,with a cleaner graphene sheet 120 having a higher electron mobility. Inone embodiment a 10 micron by 10 micron square graphene sheet 120 isused, with an impurity density level of 10⁹/cm², corresponding to anelectron mobility of about 500,000 cm²/V/s. In other embodiments thedetector may work adequately with a graphene sheet having significantlylower electron mobility, e.g., electron mobility as low as 1,000cm²/V/s.

Although limited embodiments of a Josephson junction readout for agraphene-based single photon detector have been specifically describedand illustrated herein, many modifications and variations will beapparent to those skilled in the art. Accordingly, it is to beunderstood that a Josephson junction readout for a graphene-based singlephoton detector employed according to principles of this invention maybe embodied other than as specifically described herein. The inventionis also defined in the following claims, and equivalents thereof.

What is claimed is:
 1. A photon detector comprising: a graphene sheetconfigured to absorb infrared electromagnetic waves and to undergo anincrease in a temperature of the graphene sheet when a photon of theinfrared electromagnetic waves is absorbed by the graphene sheet; aJosephson junction on the graphene sheet, the Josephson junction havinga gap coupled to electrons of the graphene sheet; and a circuitconnected to two contacts of the Josephson junction, the circuitconfigured to detect a decrease in a critical current of the Josephsonjunction, the decrease corresponding to the increase in the temperatureof the graphene sheet.
 2. The photon detector of claim 1, wherein thegraphene sheet has an electron mobility of more than 100,000 cm2/V/s. 3.The photon detector of claim 1, wherein the graphene sheet has anelectron mobility of about 1,000 cm2/V/s.
 4. The photon detector ofclaim 1, wherein the graphene sheet substantially has the shape of arectangle, the rectangle having a length and a width, the length beinggreater than or equal to the width.
 5. The photon detector of claim 4,wherein the length of the rectangle is less than 20 microns.
 6. Thephoton detector of claim 4, wherein the product of the length of therectangle and the width of the rectangle is less than 1000 squaremicrons.
 7. The photon detector of claim 1, comprising a first layer ofhexagonal boron nitride immediately adjacent a first surface of thegraphene sheet, and a second layer of hexagonal boron nitrideimmediately adjacent a second surface of the graphene sheet.
 8. Thephoton detector of claim 7, wherein each of the first layer of hexagonalboron nitride and the second layer of hexagonal boron nitride has athickness greater than 4 nm and less than 40 nm.
 9. The photon detectorof claim 1, wherein the circuit comprises a current source connected tothe two contacts of the Josephson junction and configured to drive aconstant bias current through the Josephson junction.
 10. The photondetector of claim 9, further comprising an amplifier connected to thetwo contacts of the Josephson junction.
 11. The photon detector of claim10, further comprising a matching circuit connected between the twocontacts and the amplifier.
 12. The photon detector of claim 11, furthercomprising a pulse detector connected to the amplifier, the pulsedetector comprising a voltage reference and a comparator configured tocompare a signal at an output of the amplifier to a voltage at an outputof the voltage reference.
 13. The detector of claim 1, wherein thegraphene sheet consists of a single atomic layer of graphene.
 14. Thedetector of claim 1, wherein the graphene sheet comprises two atomiclayers of graphene.
 15. The photon detector of claim 1, furthercomprising a refrigerator configured to cool the graphene sheet to atemperature below 4 K.
 16. The photon detector of claim 15, wherein therefrigerator is a pulse tube refrigerator.
 17. The photon detector ofclaim 15, wherein the refrigerator is a Gifford-McMahon cooler.